Multi-carrier Power Amplifier (MCPA) technology is being developed for a variety of wireless communication system types, including systems based on GSM and CDMA standards. For example, ERICSSON AB manufactures an RBS 6000 family of radio base stations, which include one or more shared radio units (RUSs), providing multicarrier amplification for a wide variety of radio technologies (GSM, WCDMA, LTE, etc.). Before the advent of MCPA, base stations used individual power amplifiers for the various carrier signals being transmitted, or at least used different power amplifiers for different carrier frequencies and types. In contrast, multiple carrier signals of different frequencies and, possibly, different modulation formats, can be “summed” together in either the digital or analog domains, to form a composite signal for power amplification by an MCPA. The MCPA correspondingly is configured with a sufficient amplifier bandwidth and an overall power rating to provide power amplification for the composite signal.
This capability allows several base station transmitters, each outputting a distinct carrier signal for power amplification and transmission, to use the same MCPA. The MCPA thus may be understood as a wideband power amplifier having multiple signal inputs that are combined for overall power amplification. Further, the MCPA includes or is associated with gain setting or control circuitry, providing for ongoing power control of the individual input signals. Using this arrangement, a given base station will commonly include a number of baseband units and a number of radio units. Each radio unit includes an MCPA. Correspondingly, each baseband unit includes at least one transmitter, with each such transmitter outputting a carrier signal at or otherwise corresponding to a given carrier frequency, for power amplification by an assigned one of the MCPAs.
The power limit of each MCPA restricts the number of transmitters that can be assigned to it. A typical approach to “dimensioning” a base station's MCPAs sets the number of transmitters assigned to a given MCPA based on the expected power requirements of the carrier signals from those transmitters (which may be evaluated based on worst-case peak power requirements, or based on expected average power requirements). For example, a 60 Watt (W) MCPA cannot support the worst-case power requirements of two transmitters having a maximum power requirement of 45 W each, but it may be able to support them with the assumption that they will not simultaneously require full power amplification. Thus, the two transmitters may be assigned to the MCPA, perhaps with provisions for power-clipping during instances when the actual combined power requirements of the two transmitters exceed the 60 W capabilities of the MCPA.
The transmit power of an MCPA is typically defined by the combination of average power capability (for example 60W) and peak power capability. The peak power can instantaneously be, for example, 6 dB higher than the average power and is used to handle peak-to-average variations of the composite signal. The peak-to-average ratio depends on the number of carriers and the modulations used. As an example, a single carrier with GMSK modulation has 0 dB peak-to-average ratio while the peak-to-average ratio of two 8-PSK carriers is around 6.2 dB.
Of course, clipping cannot be overly aggressive, or transmit signal quality suffers. Thus, dimensioning according to this approach essentially requires the base station designer to dedicate a given MCPA to a given number of base station transmitters, with appropriate matching by the designer of each MCPA's power capacity to the expected power requirements of the transmitters assigned to it.